Ultrasonic strain gage using a motorized electromagnetic acoustic transducer

• US 6,311,558 B1
• Filed: 03/23/1999
• Issued: 11/06/2001
• Est. Priority Date: 03/23/1998
• Status: Expired due to Fees

• Current Assignee: THE GOVERNMENT OF THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY OF COMMERCE, The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: THE GOVERNMENT OF THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY OF COMMERCE
• Inventors: Nguyen, Thanh, Hehman, Christopher, Coakley, Kevin, Alers, George, Clark, Alfred V.
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): FAYYAZ, NASHMIYA SAQIB
• Application Number:
  US09/274,538
• Time in Patent Office: 959 Days
• Field of Search: 736/43, 735/98, 735/97
• US Class Current: 73/643
• CPC Class Codes: G01L 1/12 by measuring variations in the magnetic properties of materials resulting from the application of stress
• CPC Class Codes: G01N 2291/011 Velocity or travel time
• CPC Class Codes: G01N 2291/012 Phase angle
• CPC Class Codes: G01N 2291/02827 Elastic parameters, strength or force
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0425 Parallel to the surface, e.g. creep waves
• CPC Class Codes: G01N 29/227 related to high pressure, tension or stress conditions
• CPC Class Codes: G01N 29/2412 using the magnetostrictive properties of the material to be examined, e.g. electromagnetic acoustic transducers [EMAT]; investigating the presence of flaws using eddy currents G01N27/90, magnetostrictive transducers B06B1/08, measuring magnetostrictive properties G01R33/18
• CPC Class Codes: G01N 29/265 by moving the sensor relative to a stationary material

Abstract

A method for measurement of stress in a specimen utilizing a motorized electromagnetic acoustic transducer (EMAT). Stress causes a rotation of the pure-mode polarization directions of SH-waves and a change in the phase of waves polarized along these certain directions. The method utilizes a rotating small-aperture EMAT, connected to a processor, to measure phase and amplitude data as a function of angle. The EMAT is placed on a workpiece at the location where the stress is to be measured. The acoustic birefringence B is determined from the normalized difference of these phases. From these data, an algorithm calculates values of B. and φ. The workpiece is then stressed or its stress state is changed. The values are measured again at the same location. Stress is determined from the change in B and φ.

Claims

1. A method for measurement of stress in a specimen comprising the steps of:

placing a rotating EMAT at a first location on a specimen, said specimen being in an unstressed state;

controlling said rotating EMAT to emit a first toneburst which creates shear horizontal SH waves in said specimen;

measuring a phase of a received signal echo from said specimen;

measuring a first time of flight (TOF) from said received signal echo to select a zero crossing in the signal echo;

storing said first TOF and determining an orientation of pure mode polarization directions φ

of the specimen, where φ

is an angle of rotation from pure mode polarization directions;

inducing a stressed state in said specimen;

said stressed state having a σ

_xx component, a σ

_yy component and a σ

_xy component;

using said rotating EMAT at the first location at a later time on said specimen, causing said rotating EMAT to emit a second toneburst;

measuring a phase of a second received signal echo from said specimen;

measuring a second time of flight (TOF) from said second received signal to select a zero crossing in the second signal echo;

comparing said first TOF and said second TOF and determining an orientation of pure mode polarization directions φ

of the specimen in the stressed state, where φ

is an angle of rotation from pure mode polarization directions; and

calculating a change in stress component σ

_xx, stress component σ

_yy, and stress component σ

_xy in said specimen using φ and

the first and second TOF'"'"'s for the unstressed state and for the stressed state, respectively.

2. The method for measurement of stress in a specimen as in claim 1 further comprising the steps of:

rotating said rotating EMAT with a motor controlled by a computer.

3. The method for measurement of stress in a specimen as in claim 2 further comprising the steps of:

mounting said rotating EMAT to the specimen by a magnetic connector.

4. The method for measurement of stress in a specimen as in claim 3 further comprising the steps of:

storing said first TOF and said second TOF in a computer storage media.

5. The method for measurement of stress in a specimen as in claim 4 further comprising the steps of:

displaying the change in stress components σ

_xx, σ

_yy, σ

_xy on a visual display device.

6. The method for measurement of stress in a specimen as in claim 2 further comprising the step of:

coupling said motor to said rotating EMAT with a gear drive.

7. The method for measurement of stress in a specimen as in claim 2 further comprising the step of:

coupling said motor to said rotating EMAT with a chain drive.

8. The method for measurement of stress in a specimen as in claim 2 further comprising the steps of:

receiving a SH-wave generated by said rotating EMAT in said specimen;

said SH-wave having an x and y component with amplitude sin θ and

cos θ

, respectively for various values of θ

, where θ

is an angle of rotation of said rotating EMAT from an acoustic axis in said specimen in an unstressed state, effected by rotating said rotating EMAT with said motor on said specimen;

calculating a particle velocity U from the equation;